The present invention relates to a flexographic rotary platen printing press utilized for printing indicia onto a flexible web, and, more particularly, to a flexographic rotary platen press having a printing system that makes the printing process easier and with accurate indexing of multiple printing colors.
The use of various printing presses are, of course, well know for their use in providing the printing of a web of flexible material, such as plastic material used in packaging industry for various products. In such systems for printing and in general, the packaging machine, or host machine, actually pulls the web through the print station and that host machine therefore normally provides the movement of the web as well as the registration to insure that the indicia is printed on to the web at the proper location along that web and that each of that indicia is properly spaced along the web.
One typical system currently used for such purpose is the platen press where the web is positioned on the platen and held motionless while a generally planar printing plate is impressed against that portion of the web, sandwiching the web material between the printing plate and the platen. While that Type of system allows good registration of the indicia to be printed onto the web, there is an inherent fault in that it is extremely difficult to maintain an even print over such as large area, that is, the printing plate must be precisely planar and parallel to the plane of the platen or the eventual printed indicia is uneven, and exhibits a non-uniform pattern.
Another current system is with the use of a flexographic press where the web is continually in motion and is moved along its longitudinal axis by the various mechanisms of the host machine. In that system, there is a rotary print drum that is rotated and the web is moved intermediate that rotating printing drum and a round impression roller as opposed to the flat platen of the previous platen press. The cylindrical rotary print drum has the desired indicia on a printing plate located on the external peripheral surface of that printing drum and rotates in the same direction as the web. An inking system is used to apply the ink to the printing roller and that inking system is well know and uses a anilox roller to apply the ink to the particular indicia on the print drum.
Again, however, the aforedescribed system has certain limitations as the moving web must be carefully aligned with the platen and the printing roller and thus is carried through a large number of idler rollers that guide the web to the proper position for printing, including an impression roller that requires the web to form a complete S configuration for correct alignment, registration and for providing the proper tension at all times on the moving web.
Thus, with the moving web type of printing press, the system requires a large number of tensioning and idler rollers to hold the web in the correct alignment and to create the proper tension in the web as it moves in a serpentine fashion through the flexographic printing press. Each roller, therefore, changes the direction of the web as it is moving through the printing press and each roller and change of direction also induces a stress on the web and can cause a stretching of the web and result in the final printed indicia being blurred. Accordingly, while the use of a multiplicity of tensioning and other rollers is necessary in the operation of the aforedescribed press to carry out printing on the fly, there is also a downside, that is, while the rollers are essential in the overall control of the web tension and control of the web orientation, the use of so many rollers generally creates many changes in direction of the web and also introduces stresses in the web that are undesirable.
The problem is aggravated with the use of multicolor printing where different colors are used since those different colors are printed on to the web serially, that is, after the printing of one color on to the web, the web, still moving at a high speed, progresses to a further printing cycle where the next color is applied to the web and so on. Obviously then, with a multi color printing, there are two, three of additional sets of rollers through which the web must travel and, again, each additional set of rollers can cause stress, stretch the web and make the registration of the subsequent colors extremely difficult. Too, with the multiple color printing presses, there is no room to insert dryers intermediate the color printing steps and thus the system is limited to materials having some porosity of the web material to allow the ink to dry before passing through a later printing stage with a different color. Accordingly, the use of such a multi-color rotating drum printing press is inappropriate for the other non-porous materials of webs, such as polyethylene, nylon, or polypropylene.
Thus, it would be advantageous to provide a flexographic printing press that can print the desired indicia upon a web without an excess of tensioning rollers or S-bends, be able to be used on non-porous materials and yet achieve a good printing quality and accuracy of the position and registry of the printed material.